A certain amount of vibration or shock occurs naturally as a dynamic loading condition in an aircraft turbine engine, and contemporary aircraft turbine engines are designed to accommodate the naturally occurring dynamic loading conditions. However, occasionally, an extreme loading condition occurs. Extreme loading conditions may induce a large impact within the aircraft turbine engine and result in a prolonged unbalanced load. An example of an extreme loading condition is a fan blade out event (wherein there is a loss of a fan blade). In the case of a fan blade out event, the segment of the turbine engine that bears the brunt of the impact is typically a composite structure referred to as the fan case.
Generally, a fan blade out event causes a heightened, momentary and direct, increase in transmitted load to the fan case, followed by an unbalanced load. The momentary and direct increase in transmitted load may cause damage, such as a crack, tear, dent, or puncture in the fan case. The resulting unbalanced load may start or exacerbate damage, for example, by inducing damage propagation within or along the fan case for the remainder of aircraft airborne time. Damage propagation, if left unchecked, could result in loss of the aircraft inlet. Therefore, a turbine engine fan case must be designed to tolerate and survive extreme loading conditions such as fan blade out events.
Currently, various methods are employed to design composite structures, such as turbine engine fan cases, that tolerate and survive fan blade out. For example, some designs reinforce a composite structure with added layers of metal or composite materials. However, as with many aspects of aircraft design, there is constant pressure to minimize weight, and the extra layers of metal or composite materials for strengthening may unacceptably increase the mass and the weight of the composite structure.
Accordingly, an improved composite structure and method for making same is desirable. The improved composite structure has an identified reinforcement region and employs locally strengthened areas therein. The locally strengthened areas within the reinforcement region have load distribution devices that redistribute load in order to (i) locally strengthen an area around damage induced by the initial momentary and direct transmitted load, and (ii) limit propagation of the damage induced by the initial momentary and direct load transmitted load during a subsequent unbalance load. The improved composite structure reduces the impact of the fan blade out phenomenon in a weight efficient manner.